Duplicate users showing in the "groups" list

Been asked to post this in the forums NOT the bug list.
after a migration from MS
we found that users in the groups list can be duplicated, but not for all users.

so if we have a list for “domain users”
and we want to add more users to this group, we use the add button,

but we find users in both the list AND the add list.

so if we have a user “bob”, “jane”, “mary”

and “bob” & “jane” are in the domain users" group.

we then go to the “add” button" to add users but “bob” is in the list to “add” but “jane” is not
indicating she is already added…
We can re-add “bob” again into the “domain users” and it shows as TWO IDENTICAL accounts for “bob”

however when inspecting the “users” in the system there is only ONE “bob”

it is as if the character searching is not filtering correctly and correctly removing users already IN the
“domain users”,but it does NOT do it for every user.
we strongly suspect that there is some sort of issue related to the filtering & multi byte charactersets.